1. Field of the Invention
The invention concerns a tension device comprising a tension roller which is supported and free to rotate on a supportive member, a pivot bearing outside an axis of rotation of the tension roller and capable of being locked in position with respect to an engine block or the like, a tension spring pressing the tension roller, for adjustment, against the drive belt or the like, and of an additional spring pressing against the drive belt.
2. Description of the Prior Art
A tension device of the aforementioned type has already been disclosed in DE-OS No. 2,617,368. In this design a swiveling lever is pivotably supported on a supporting plate. A tension roller is arranged outside a pivot bearing and is tensioned against a drive belt by a working spring acting on the other end of the swinging arm. This working spring is constantly active during operation. To permit an optimal basic adjustment, the supporting plate is for its part connected with the engine block via another pivot bearing and may be fixed in the proper place by a fastening screw. An additional tension spring arranged between the engine block and the supporting plate permits the basic adjustment to be made with constant tension against the drive belt when the tension roller is arrested in its swivel motion with respect to the supporting plate. After release of the arrest, operating shocks and swings of the drive belt are absorbed and equalized by the working spring. Assembly of the tension device in the manner described is relatively costly and, due especially to the arrest required, complicated. It requires a complicated procedure. In addition, the known tension device has a great range of adjustment of the swiveling lever, which may result in damage having serious consequences. Strong pulse shocks in the operating behavior of the engine cause the drive belt to stretch severely in the region of the tension device. This leads to deflection of the tension roller in the direction away from the drive belt. Since the known tension device exhibits no damping of any kind in this direction of motion, the drive belt may be slack for at least a short time. In this phase the likelihood of slippage between the drive wheels and the drive belt is very great. In this case, especially in toothed belts, a failure to maintain a correct angle of rotation between, for example, crankshaft and cam shaft occurs due to skipping of one or more teeth, which may lead to shutdown of the engine, and at the least to defective operation.